Apparatus to establish and optimize sedimentation and methane fermentation in primary wastewater ponds

ABSTRACT

A method of disposing waste includes the step of forming a primary waste pond called an Advanced Facultative Pond (AFP). A stable microbiological methane fermentation zone is established within the AFP. The system constitutes a complete primary treatment of organic waste and wastewater that does not require daily sludge handling typically associated with organic waste treatment and disposal. The method of the invention converts organic compounds, including settleable solids, into methane. The invention controls sulfide odors from methane fermentation. The invention also provides a method of filtering raw wastewater through a bed of fermenting organic solids suspended by gas evolved in a fermentation zone. Hydrogen sulfide is oxidized in accordance with the invention. The technique of the invention biologically increases pH near the pond surface, thus retaining hydrogen sulfide in solution in the pond water. The biological increasing of the pH level near the pond surface increases the rate of die-away of pathogenic bacteria. The invention transforms proteins and other organic nitrogen compounds to nitrogen gas. The invention also transforms proteins and other organic nitrogen compounds to nitrogen gas; the invention detoxifies chlorinated hydrocarbons and volatile organic compounds; the invention captures and stores gases evolved from methane fermentation; the invention removes heavy metals, while establishing meromixis in fermentation cells or zones within primary wastewater ponds.

This is a divisional of application Ser. No. 09/552,576, filed Apr. 19,2000 now U.S. Pat. No. 6,852,225.

This application claims priority under 35 U.S.C. §119 (e) to U.S.provisional patent application Ser. No. 60/130,210 filed Apr. 20, 1999,which is hereby incorporated by reference in its entirety.

BRIEF DESCRIPTION OF THE INVENTION

This invention relates generally to waste treatment. More particularly,this invention relates to a new method for first stage liquid wastetreatment that is simpler, safer and less costly than current methodsused for primary wastewater treatment.

BACKGROUND OF THE INVENTION

The first, or primary stage of conventional liquid born waste treatment,as currently practiced, consists of sedimentation of heavy inorganicdetritus called grit, followed by floatation of light materials calledfloatables and congealed fat called grease, and sedimentation of heavierorganic solids called sludge. Grit is generally disposed of by burial ona day by day basis. Sludge is usually taken off in a side stream andtreated in a separate sludge digester where its organic content ispartially converted to carbon dioxide, methane, and inert gases, and theresidue is de-watered and disposed of, usually by burial. Floatablematerial along with grease is either finely ground and introduced to aseparate sludge digester with sludge or disposed of separately by burialor incineration. Under new pollution control legislation, grit,floatable materials and fresh or partially stabilized sewage sludge,unless heat treated or heavily disinfected with chlorine, are regardedas highly infectious and potentially toxic or hazardous. Thus, to handleand dispose of them safely is extremely expensive.

In view of the foregoing, it would be highly desirable to provide animproved technique for wastewater treatment. Ideally, the system wouldbe low cost and would not require frequent waste handling.

SUMMARY OF THE INVENTION

A method of disposing waste includes the step of forming a primary wastepond. A stable microbiological methane fermentation zone is establishedwithin the primary waste pond. The system constitutes a complete primarytreatment of organic waste and wastewater that does not require dailysludge handling typically associated with organic waste treatment andsludge disposal. The method of the invention converts organic compounds,including settleable solids, into methane. Carbon dioxide is alsoproduced and used by algae. The invention controls sulfide odors frommethane fermentation. The invention also provides a method of filteringraw wastewater through a bed of fermenting organic solids suspended bygas evolved in a fermentation zone. Hydrogen sulfide is oxidized inaccordance with the invention. The technique of the inventionbiologically increases pH near the pond surface, thus retaining hydrogensulfide in solution in the pond water. The biological increasing of thepH level near the pond surface increases the rate of die-away ofpathogenic bacteria. The invention transforms proteins and other organicnitrogen compounds to nitrogen gas. The invention also detoxifieschlorinated hydrocarbons and volatile organic compounds. In variousembodiments, the invention further captures and stores gases evolvedfrom methane fermentation; and removes heavy metals, while establishingmeromixis in fermentation cells or zones within primary wastewaterponds.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference should be made tothe following detailed description taken in conjunction with theaccompanying drawings, in which:

FIGS. 1A-1E illustrate a waste treatment apparatus in accordance with anembodiment of the invention.

FIGS. 2A-2E illustrate a gas collection fermentation cell in accordancewith an embodiment of the invention.

Like reference numerals refer to corresponding parts throughout thedrawings.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the invention, the drudgery, hazards, and cost of dayby day handling of grit, floatables, grease, and sludge are eliminatedfor periods of up to 20 years.

Since burial is likely to be the ultimate method for disposal offloatables, grease, grit, and sludge, the principle of this new methodis to create a combined treatment and disposal site at the point ofwaste introduction.

In structure, this new method is carried out in two ponds—one within theother. The larger of the two surrounds and overlays the smaller pond. Itis called the outer pond. The other pond is located in the bottom of theouter pond and is called the inner pond or pit. The outer pond isdesigned to contain aerobic or semi-aerobic wastewater. The inner pondis designed to prevent intrusion of dissolved oxygen from the outer pondand contain semi solid slurry in the highly reduced anaerobic stateneeded as substrate to foster methane fermentation. The inner pond isalso designed to foster sedimentation of solids as well as theirconversion to methane.

Because dissolved oxygen is inhibitory to methane fermentation, thefermentation zones, in one embodiment of the invention, are designed tobe 1 to 10 meters deep, preferably 2 to 8 meters deep, and mostpreferably 3 to 5 meters deep with high walls to prevent intrusion ofdissolved oxygen from the outer pond by convection or by wind drivencurrents. Raw, unscreened, unsettled sewage is introduced to this pitthrough a downdraft pipe, also called an influent pipe, its openinglocated near the bottom of the pit. In design, the pit volume isdetermined from the projected volume of inert materials such as grit,silt and sludge residue to accumulate over 20 years plus a plenum volumerequired for long term methane fermentation of the organic load based onprojected loadings and temperatures within the pit.

The outer pond is designed to grow algae, which by their sunlight andheat absorption, create a warm surface layer of water. Algae, growingnear the surface, also produce surplus oxygen, thus controlling odor,raising the pH of the water by extracting carbon dioxide, thus enhancingdisinfection, and tending to precipitate metals.

To initiate the process, the pit is seeded with several tons of activemoist sludge from a methane producing culture, thus assuring thepresence of a large number of methane bacteria. Wastewater is thenapplied to the pit at a rate proportional to the organic load andtemperature. One or more hydraulic equalization vents are provided toprevent washout of the pit walls and to equalize the water depth in thepit and the outer pond. The outer pond is designed to retain wastewaterfor 10 to 20 days depending on climate; a smaller volume is used forwarmer climates and a larger volume is used for cooler climates. Theoverflow of the outer pond is located so that it is also 2 to 6 metersdeep, and preferably 3 to 4 meters deep and extends upward 1 to 1.5meters above the upper edge of the wall surrounding the pit. Althoughthe overflow invert determines the depth of the outer pond, the wateroverflowing should be drawn from a depth of about 1.5 meters to avoidentraining floatable materials. This is done by extending the intake ofthe outlet pipe downward into the pond to a depth of 1.5 meters. Becausematerials that float are accumulated in the primary pond, their removalis provided for by installing a paved beach-like scum ramp along theedge of those portions of the pond where wind driven floating materialis likely to accumulate, typically at the water's edge of the outerpond. This material, which generally consists of grease and plasticmaterials is quite inert, dries on the beach, and can be removedperiodically by mechanical means for burial. Although it may beinfectious or hazardous, very little odor or fly breeding is associatedwith this material due to its inert nature. Further, the fact that thescum ramp is sufficiently flat allows the floatables to dry out quicklyand encourages “beaching” of the floatables by small waves.

It is important to further explain the physical, chemical and biologicalactivity in the pits. As sewage solids enter and tend to settle, theysettle through an intensely anoxic zone which contains facultativeheterotrophic bacteria and methane bacteria. The interaction of thesetwo types of bacteria is well known from experience with conventionaldigesters and septic tanks. Facultative organic acid forming microbesproduce primarily acetate, CO₂, and hydrogen which are then transformedto methane by the methane bacteria. The heterotrophic and methanebacteria adhere to surfaces which are abundantly provided with theretained sewage solids. There are several unique characteristics ofthese pits not shared by ordinary sewage sludge digesters. First,because sewage sludge digesters are expensive, their size is such thatsludge is only retained for 20 to 40 days, and therefore digestion is,at best, only partially complete, and its infectious nature onlyminimally eliminated. Conventional sludge digesters operate on a sidestream of sludge so there is no contact between fermenting sludge andthe raw waste. The pits of the instant invention are inexpensive sotheir residence time for the continuously fermenting and consolidatingof settleable solids can be literally hundreds of days. Consequently,adaptation of microbes to unique wastes and sub optimum temperatures ispossible, and fermentation of deposited organics can continue until onlytheir inert residues remain. Such long residence time also is lethal tothe ova of parasites and to other pathogens. All of the wastewaterpasses through and contacts this anoxic zone, before entering the outerpond. For practical purposes, all solid settleable material simplyremains where deposited until decomposed to the point that only inertmaterial remains. The volume of this inert material is remarkably small,amounting to less than 5 liters per person per year. As a result, manyyears are required before residual sludge removal is needed.

The gases emitted by the pits also differ significantly from thatemitted by conventional digesters. Conventional digester gas usuallycontains about 60% methane, about 30% CO₂, and small amounts of H₂S andthe balance N₂, and other inert gases. The gases emitted by the anoxicpits of the invention contain about 70%-85% methane, with the balancebeing mostly nitrogen, and small amounts of CO₂, and other inert gases.The difference in gas composition is believed to result from the morecomplete fermentation that occurs in the pits plus the fact thatoverlying water in the peripheral pond absorbs most of the CO₂ and H₂Sproduced in the anoxic zone. The CO₂ is converted to bicarbonate in thealkaline high pH surface waters and H₂S is converted to the HS-ion andeventually to sulfate as it emerges toward the outer pond surface. Theouter pond near its surface is highly aerobic—its free molecular oxygencontent either resulting from the growth of microalgae in the layer ofwater above the pit walls or from re-circulation of oxic waters from ahigh rate pond in series with the pond being disclosed, or in rarecases, when needed, by supplementary mechanical aeration.

Functionally, the pits are highly anoxic and strongly reducing.Carbohydrates are quickly converted to methane. Proteins and amino acidare hydrolyzed by proteolytic enzymes, and ammonium is released to beconverted to nitrate and N₂ gas via heterotrophic nitrification andde-nitrification. Lipids are converted to glycerides and then tomethane. Some toxic substance such as certain chlorinated hydrocarbonsare dissociated and destroyed. Heavy metals such as lead, chromium, andothers combine with sulfides in the fermentation zone and are retainedas precipitates along with other inert residuals.

Organic and inorganic sludges with adhering gas tend to be lifted by gasbuoyancy, but as the mixture rises toward the level of the top of thepit walls, the adhering gas bubbles expand in size and break away fromthe sludge and rise alone to the surface. The sludge resettles passingdownward through any new sewage entering the system. This continuouslyworking sludge-bed has a filtering effect on new particles and dissolvedorganics entering the system tending to carry them downward and toretain them within the sludge blanket. Microbes adhering to thesettleable solids adsorb soluble organics as well as colloidalparticulates, thus reducing soluble and colloidal biochemical oxygendemand (BOD). Also, since the column of water above the pit bottom is6-8 meters (18-25 feet), a pressure of 500-600 grams/cm² (slightly over7 psi) compresses loose solids tending to increase their density andhence their rate of sedimentation. Accordingly, many substances whichwould float if deposited at the surface do not float when injected intothe sludge blanket. Individual pits should ideally be 0.09 hectare andnot be larger than about 0.1 hectare (¼ acre) so when larger areas areneeded to maintain load criterion, several pits should be used. A cone,or flow deflector, is used to assure distribution of sludge over the pitbottom area. Because it may be necessary to de-water and clean pitsafter 20 years, more than one pond pit system in parallel should beused. When greater than one pond pit system is used, the ponds should befed by way of a hydraulic head equalizing distributor 41 on the influentpipe 12. Moreover, equilateral bifucation of the influent pipe 12 viathe hydraulic head equalizing distributor permits equal loading of up tofour pits within one outer pond 15. In large systems, several pits orfermentation cells would be aligned in multiple parallel fermentationtrenches running transfer to the prevailing wind. In this configuration,each individual pit would be separated by a baffle so that its surfacearea would be no greater than about 0.1 hectare.

The general nature of the invention has now been described. Attentionnow turns to a more detailed discussion of different embodiments of theinvention. The key to successful methane fermentation is theestablishment of protected anoxic zones where naturally presentheterotrophic and methane bacteria flourish, within primary waste ponds,sometimes referred to as Advanced Facultative Ponds (AFP). The zones arecreated by preventing the intrusion of cold water containing dissolvedoxygen. This protection can be achieved economically by isolating thefermentation zone with a surrounding wall or other vertical structure toprevent the intrusion of cold, oxygen-bearing water. Crucial factors forthe special zones of stable methane fermentation are: depth, surfacearea, volume, the type and location of the inlet structures, the heightof the current deflector or surrounding wall, the location of specialvents, hydraulic and organic loading rates, the exclusion of storm waterand inert solids or grit, and the residence time of the settled solids.The advantages of in-pond methane fermentation include the eliminationof the need to remove, handle, and dispose of sludge residuals over longperiods of time, usually several decades; the purification andenrichment of methane gas emerging from the zones of fermentation; thereduction, precipitation, and removal of heavy metals; theimmobilization of parasites; the bio-degradation of many toxic organiccompounds; and, significant removal (60% or more) of BOD.

FIGS. 1A-1E illustrate the system of the invention. FIG. 1A is a topview of the inner pond or pit 10. The inner pond 10 generally includes aflow deflector 11, an influent pipe 12 and an oxygen deflector 13. Theoxygen deflector should be on the entire periphery of the inner pond 10.FIG. 1B represents a cross-sectional view of the Advanced FacultativePond 14, which comprises both the inner pond 10 and the outer pond 15.The outer pond 15 may be any shape, but typically is either round orrectangular. Also shown within FIG. 1B are the flow deflector 11 and theinfluent pipe 12. Although a single inner pond 10 is shown, it is to beunderstood that there may be more than one inner pond 10 in eachadvanced facultative pond 14. All inner ponds 10, however, must belocated away from any floating or surface aerators 40. As previouslynoted, supplemental mechanical aeration is sometimes needed when, due tolight or temperature inhibition, there is insufficient photosynthesis tocontrol odors. Aerators 40 must be located outside and at least 10meters from the outside edge of the fermentation pits. In a preferredembodiment, floating surface aerators of the injection type are usedsince they create no aerosols and are very quiet. Their flow direction,however, must be away from the fermentation pits. The number and size ofthe aerators is determined by the organic load coming from thefermentation pits. Moreover, the location of the inner pond 10 may bevaried within the Advanced Facultative Pond 14. FIG. 1B further shows aspecial influent structure 41 designed to permit only design flow orless to enter the fermentation zones. FIG. 1C represents a more detaileddescription of the inner pond 10 within the Advanced Facultative Pond14. Specifically depicted are the flow deflector 11, the influent pipe12, the oxygen deflector 13, the hydraulic pressure equalization vents17, also called an overflow pipe, the deflector membrane 18, the bufferzone 19, the settling zone 20, the fermentation zone 21, the highdensity polyethylene (HDPE) lined walls 22, which forms a strong, thickplastic sheeting used to line ponds to keep them from leaking outward,the slope of which will depend upon local soil conditions, and aconcrete floor reinforced with No. 6 reinforcing bars 23, which can bemade of any non-corrosive material, including, but not limited tostainless steel and fiberglass. Storm flows, which usually containdissolved oxygen, are directed to the outer pond 15 by a special bypassin the head works within the influent pipe 12. In a preferredembodiment, the hydraulic pressure equalization vent 17 is at a rightangle to the prevailing winds to avoid wind induced pressuredifferentials from causing back flow of oxygenated water that couldimpair fermentation. For the particles and gases moving through thebuffer zone 19, the settling zone 20 and the fermentation zone 21, in anembodiment of the invention, the maximum upflow velocity should be 1.8meters per day. The buffer zone 19 represents the top layer of thefermentation pit where gases are rapidly traveling to the surface. Thesettling zone 20 represents where solid particles that were travelingupward with the gases are released from the gases and held back byfrictional drag and eventually re-settle in the fermentation zone 21.The fermentation zone 21 is where the facultative and methane bacteriaexist. FIG. 1D represents a detailed depiction of the oxygen deflector13. In a preferred embodiment, the diameter of the inner pond 10, whichis surrounded by the oxygen deflector 13 should not be greater than 33square meters in order to maintain the upper aperture area of less than0.1 hectare. Larger areas would be prone to mixing during strong winds.The oxygen deflector 13 is composed of heavy wall, polyvinyl chloride(PVC) posts, or similar material. Overall, the oxygen deflector 13should be about 3 to 6 meters in length and preferably 4 to 5 meters inlength with about 2.5 meters being above-grade (above the water level).In a specific embodiment, HDPE should be woven around and fastened tothe oxygen deflector 13. FIG. 1E represents the system when inner andouter ponds are placed in parralel.

FIGS. 2A-2E illustrate a gas collection fermentation system of theinvention. FIG. 2A represents a top view of the inner pond 10. Innerpond 10 generally includes a flow deflector 11, an influent pipe 12, anoxygen deflector 13, a gas deflector canopy 28, also known as asubmerged gas collector, a gas fueled heat power generator 42 and a agas cap 31. In a preferred embodiment of the invention, the gasdeflector canopy 28 is composed of a reinforced plastic membrane. Theuplift of gas pressure under the canopy requires that the canopymaterial must be reinforced with strips of webbing to keep it fromtearing. These strips extend from the periphery of the oxygen deflector16 to the canopy vertical positioning float 37. The oxygen deflectorshould be on the entire periphery of the inner pond 10. FIG. 2B, similarto FIG. 1B, represents a cross-sectional view of the AdvancedFacultative Pond 14, which comprises both the inner pond 10 and theouter pond 15. Also shown within FIG. 2B are the flow deflector 11, theinfluent pipe 12 and a central mast pipe 29 used for gas transport. In apreferred embodiment, the pipe is made of stainless steel. As with FIG.1B, although a single inner pond 10 is shown, it is to be understoodthat there may be more than one inner pond 10 in each advancedfacultative pond 14. All inner ponds 10, however, must be located awayfrom any aerators. Moreover, the location of the inner pond 10 may bevaried within the Advanced Facultative Pond 14. FIG. 2C represents amore detailed description of the inner pond 10 within the AdvancedFacultative Pond 14. Specifically depicted are the flow deflector 11,the influent pipe 12, the oxygen deflector 13, the hydraulic pressureequalization vent 17, the deflector membrane 18, the buffer zone 19, thesettling zone 20, the fermentation zone 21, the HDPE lined walls 22, theslope of which will depend upon local soil conditions, No. 6 Rebar 23, agas deflector canopy 28, a central mast pipe 29, a heavy wall containinga heating coil or fuel cell 30 and a gas cap 31. In a preferredembodiment, the heating coil within the heavy wall 30 is 2.5 cm indiameter. FIG. 2D represents a detailed depiction of the gas cap 31.Specifically, the gas cap 31 comprises a gas outlet 32, a flange cap 33,a top stop ring 34, a bottom stop ring 35, a cap roller 36 and a canopyvertical positioning float 37, also known as a buoyant ring. The flangecap 33 is gas tight and is sealed after the top stop ring 34 is inplace. Moreover, the top stop ring 34 is put in place after the flangecap 33 is in place. FIG. 2E depicts a top view of the gas cap 31.Specifically depicted are a gas cap lower skirt 38, a gas cap main body39, a cap roller 36, a central mast pipe 29 and a top stop ring 34. Thegas produced from the fermentation pits travels along the central mastpipe and is collected in the gas cap. The collected gas can then bepassed through the gas outlet 32 to, for example, a power generator forthe production of electricity.

The first reactors in the system of the invention are the speciallydesigned fermentation zones within two or more Advanced FacultativePonds (AFP). The purpose of the fermentation zones is to permit completesedimentation of the settleable suspended solids and theirmicrobiological conversion by naturally-occurring heterotrophic andmethane-forming bacteria to water, methane, carbon dioxide, and nitrogengas. Further purposes are sedimentation and retention of parasite ova,precipitation of heavy metals, and the biodegradation of toxic organicchemicals. In a preferred embodiment, fermentation zones should be 6 to8 or more meters in depth below the AFP/water surface with a top surfacearea of not more than 0.09 hectare (900 square meters). The influentwastewater rises at a velocity in the range of 0.7 and 1.4 mm perminute. The redox potential must be in the range of −0.3 to −0.5 volts,and the pH should be in the range of 6.5 to 7.5. The 0.09-hectareaperture is necessary to reduce the probability that wind-inducedconvective currents will intrude bringing dissolved oxygen, higher pH,and positive redox potential water into the fermentation zone.

The methane forming microbes are extremely sensitive to, and areinactivated by, pH levels above 7.5 and below 6.5. If any water withadverse chemical characteristics enters the fermentation zone, themethane-forming bacteria may be inactivated and take weeks to resumetheir normal metabolic activity. The low overflow and consequently lowhydraulic loading rates in the fermentation zones are designed to permitnear complete bioconversion of volatile organic solids to gaseous andliquid products. The very low overflow velocity also increases theprobability of pathogenic parasite ova retention.

In operation, after grit removal as needed, and comminution or grinding,the otherwise untreated wastewater is introduced into the fermentationpit of the AFPs. Systems, which often having intermittent pumpedinfluent, may require carefully staged pump selection, or a surgechamber to avoid the washout of essential bacteria and settleablesolids. High storm flows in combined sewers and excessive sewerinfiltration and inflow require a special influent structure 41 designedto permit only the design flow or less to enter the fermentation zones.Higher than design flows, usually resulting from storm runoff andrelated infiltration and inflow, should be automatically bypassed intothe AFPs, thus protecting the special environment within thefermentation zone from dissolved oxygen (DO) in the influent and fromexcess upflow velocity and consequent wash out. The use of such a bypassin the fermentation zone influent system is an important innovation inpond design, although it makes use of a well-known hydraulic device; alateral overflow structure that diverts high flows from the pit to theouter pond. Influent flow dividers must be designed to avoid aerationand entrainment of air due to turbulence. Design or lesser flows areconveyed by pipe 12 into the center of the fermentation zones andreleased vertically downward approximately 1.0 meter above the tip ofthe flow deflector. Because of the low upflow velocity (<1.8 meter perday) and the compression of solids by hydrostatic pressure increasingtheir density, settleable primary solids tend to remain near the bottomof the fermentation zone creating a circular fall-out mound around andnear the flow deflector. As this mound builds up in depth, its basebecomes extremely anoxic permitting anaerobiosis culminating in robustmethane fermentation. Studies of gas emission from these zones indicatethat up to 90% of the gas produced is evolved within five meters of theinlet. Bubbles of CO₂, N₂ and CH₄ form, adhere to, and lift the solidparticles. Bubble expansion soon causes the solids to separate andre-settle while the gas rises to the surface. Separation of gas bubblesand solid particles occurs because the gas bubbles expand and accelerateas they rise, whereas the solid particles are held back by frictionaldrag and eventually drop off and resettle as the enlarging andaccelerating bubbles rise toward the surface.

These biological, chemical, and physical interactions result in ananaerobic field of rising and falling particles through which all of theinfluent wastewater must pass and by which it is coarsely filtered. Thismethod to improve in-pond methane fermentation is totally different fromconventional sewage treatment in which the settleable solids or primarysludge are separated from the influent wastewater and conveyed intoseparate sludge digesters for anaerobic digestion. Because of the highcost of separate sludge digesters, the size and volume of separatesludge digesters are based on a sludge residence time required toproduce drainable sludge, e.g. 40 days, rather than the time requiredfor complete methane fermentation. Because the fermentation zones inAFPs may be constructed of earthwork and plastic, they are sufficientlyinexpensive that they can be made large enough to permit almost infiniteretention and very complete methane fermentation of the settleablevolatile solids and to eliminate the need for the disposal of organicsludge over several decades.

Compared with conventional treatment steps of sedimentation, separatesludge digestion, and aerobic oxidation, the advantages of in-pondsedimentation and fermentation of settleable solids are: the yield ofbacterial cell material is much lower; solids concentration is notlimited by oxygen transfer; there is minimal need to provide aeration;supplementary nutrients are not usually required; and, rather thanconsuming energy for mechanical aeration, heat power or fuel cell energyproduction from the recovery of methane gas may be simply attained bymethane capture using the apparatus shown in FIGS. 2A-2E.

Beyond the flow deflector, the fermentation zone bottom is designed flatand lined with concrete sufficiently strong to support wheeled equipmentshould cleaning ever be needed. If gas collection and power generationwith the collected gas are intended, in one embodiment of the invention,the central part of the methane fermentation zone should be designed tocontain embedded heavy wall stainless steel pipes that are connected toa closed heat exchange system on the gas fueled heat power generator 42.Gas engine heat recovery provides small amounts of incremental heat toaccelerate and improve the efficiency of methane fermentation in theconcentrated bottom sludges of the fermentation zone. Once established,the media in the fermentation zone becomes more concentrated, and hencesufficiently more dense than the overlying water that the overlyingwater will not displace it even though the media is warmer than theoverlying water, a condition called meromixis. The special media andenvironment within the fermentation zone is laterally isolated fromwaters of the surrounding AFP; for example, by means of vertical walls,most economically made from plastic sheeting supported by concreteembedded posts made of plastic, wood, or composite material.

If gas capture is not intended, the top of the walls should be about oneand one third meters below the fixed water surface of the overlying AFP.This provides for an approximately one meter thick strata at the surfaceof the AFP which usually contains sufficient dissolved oxygen to act asa buffer against the release of objectionable odors that may otherwisebe present in the gases entering the atmosphere above the fermentationpit.

If gas capture and power generation are intended and included in thedesign, as may eventually be required in order to control the emissionof methane, a potent greenhouse gas, the top of the oxygen deflectorshould be 1.5 to 2.5 meters below the fixed water surface of the AFP topermit gas collection using a submerged focusing gas collector andsurface gas cap (see FIG. 2B). The 1.5 to 2.5 meter edge submergencepermits the submerged gas collector 28 to be sloped upward by thebuoyant ring 37 so that gas intercepted by the submerged gas collectorwill migrate diagonally upward to a central surface gas cap positioneddirectly above the fermentation zone. In a separate embodiment, thecentral mast pipe may be hollow for gas storage. The submerged gascollector (deflector) and surface gas cap are held in a central locationby a rigid mast of plastic, wood or other non-toxic material. Concerningother design details, the oxygen deflector should be between 1.5 and 2.5meters below the fixed water surface in the AFP to provide aphotosynthetically or mechanically induced dissolved oxygen containingbuffer strata that removes objectionable odors.

Although FIG. 1 and FIG. 2 depict a circular fermentation zone, otherconfigurations that preserve design criteria of depth, aperture orsurface area, volume, and loading rates may be used. If rectangularfermentation zones are needed to meet other design criteria, theirlength should be at right angles to the prevailing wind direction, andthey should be divided into individual cells which conform to the arealdesign criteria of 0.09 hectare.

The foregoing description, for purposes of explanation, used specificnomenclature to provide a thorough understanding of the invention.However, it will be apparent to one skilled in the art that the specificdetails are not required in order to practice the invention. In otherinstances, well known circuits and devices are shown in block diagramform in order to avoid unnecessary distraction from the underlyinginvention. Thus, the foregoing descriptions of specific embodiments ofthe present invention are presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, obviously many modificationsand variations are possible in view of the above teachings. Theembodiments were chosen and described in order to best explain theprinciples of the invention and its practical applications, to therebyenable others skilled in the art to best utilize the invention andvarious embodiments with various modifications as are suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the following claims and their equivalents.

1. A liquid waste disposal apparatus, comprising: an outer pond havingan open surface and comprising aerobic wastewater, and; at least oneinner pond disposed within and below said outer pond, wherein at leastone or said at least one inner pond comprises a bottom that is at least6 meters below said open surface and comprises semi-solid anaerobicslurry to allow methane fermentation to occur; and an influent pipeconfigured to pass liquid waste to said at least one inner pond.
 2. Theapparatus of claim 1, further comprising a flow deflector positionedbelow an exit opening of said influent pipe.
 3. The apparatus of claim1, further comprising an oxygen deflector disposed within said outerpond and configured to reduce the flow of dissolved oxygen from saidouter pond into said at least one inner pond.
 4. The apparatus of claim1, further comprising an oxygen deflector that comprises a wall thatsurrounds a periphery of said at least one inner pond.
 5. The apparatusof claim 1, further comprising an aerator in said outer pond.
 6. Theapparatus of claim 1, wherein said at least one inner pond has a topsurface area of less than approximately 0.09 hectare.
 7. The apparatusof claim 1, wherein said at least one inner pond has a depth ofapproximately 6 to 8 meters.
 8. The apparatus of claim 1, wherein saidouter pond has a depth of approximately 4 to 6 meters.
 9. The apparatusof claim 1, further comprising a gas collection system configured tocollect gas produced by a methane fermentation zone within said at leastone inner pond.
 10. The apparatus of claim 9, wherein said gascollection system comprises: gas deflector canopy disposed near a topportion of said at least one inner pond and below said open surface andhaving an apex; a central mast pipe positioned vertically in said atleast one inner pond having a bottom end disposed within a bottom ofsaid at least one inner pond and a top end above said open surface andpassing through said apex of said gas deflector; and a gas cap disposedon said top end of said central mast pipe.
 11. A liquid waste disposalapparatus, comprising: an outer pond having an open surface andconfigured to hold aerobic wastewater; and; at least one inner ponddisposed within and below said outer pond, wherein at least one of saidat least one inner pond comprises a bottom that is at least 6 metersbelow said open surface and is configured to hold semi-solid anaerobicslurry to allow methane fermentation to occur; an influent pipeconfigured to pass liquid waste to said at least one inner pond; and aflow deflector positioned below an exit opening of said influent pipe.12. A liquid waste disposal apparatus, comprising: an outer pond havingan open surface and configured to hold aerobic wastewater; and; at leastone inner pond disposed within and below said outer pond, wherein atleast one of said at least one inner pond comprises a bottom that is atleast 6 meters below said open surface and is configured to holdsemi-solid anaerobic slurry to allow methane fermentation to occur; aninfluent pipe configured to pass liquid waste to said at least one innerpond; and an oxygen deflector disposed within said outer pond andconfigured to reduce the flow of dissolved oxygen from said outer pondinto said at least one inner pond.